I think you will probably not gain much if anything with a change. But the cost of the change could be significant.
All things being equal, they usually they put taller gears with larger motors. Sorry I can't tell you the actual #'s for those models.

My friend installed an overdrive in his truck a few years ago, after he bragged about his "claimed" 20% increased mileage, we figured out his savings on paper. He would need to drive it over 100K miles just to break even! I seriously doubt his truck will last that long.